Can I use just TXD, RXD and GND lines for communications with a host MCU?

Yes, you can do that. However, you need to do either of the following:

1) Set and save AT+IFC=0,0 (no flow control), OR
2) Tie RTS and CTS on the module side to GND to imitate that these lines are always active
2.1 ) Connect DTR line (GPIO-3 ) to GND if not required for use